We purchase items from the states, each invoice can have up to 20 or 30 items on it.  Each of these items are inventory.  We get separate invoices for the duty and shipping expenses that we want to include in the overall cost of the inventory.  How do I add these costs to the inventory items.  

Currently I am posting the shipping and duty invoices to a clearing account.  Then I use a spreadsheet to apportion the extra costs to the items to determine their adjusted unit price.  When entering the vendor's bill, I use my adjusted cost to enter each inventory item, then I enter a credit to the clearing account to clear that amount and reduce the bill back down to what the vendor actually charged me.  But there must be a better way to do this.!